The present invention concerns sampling methods used within oscilloscopes and pertains particularly to random sampling with phase measurement
In oscilloscopes, three different repetitive sampling methods are traditionally employed. Sequential sampling is used in equivalent time sampling scopes. In sequential sampling systems, each sample is taken at successively longer delays after a repetitive trigger event. Alternatively, the signal repetition frequency is phase locked to the sampling signal with a small fixed frequency offset. S. Nogiwa et. al, Improvement of sensitivity in Optical Sampling System, Electronics Letters, May 27, 1999, Vol. 35, No. 11.
Non sequential sampling is used in a microwave transition analyzer. The sampling pulses and the users signal are phase locked. Each has a more arbitrary, but known frequency.
The third repetitive sampling method is random sampling. In random sampling systems, the phase of the samples is typically not related to the repetitive signal input. The position of each sample on the time axis of the oscilloscope display is obtained by measuring the phase of each sample relative to a trigger signal. See, for example U.S. Pat. No. 4,884,020. For additional background information on random electrical sampling, see, for example, U.S. Pat. No. 5,315,627, U.S. Pat. No. 4,928,251, U.S. Pat. No. 4,719,416, U.S. Pat. No. 4,578,667 and U.S. Pat. No. 4,495,586.
In optical sampling oscilloscopes sequential sampling is typically used. There have been a number of disadvantages with the use of sequential sampling. For example, there is significant jitter in phase locking of the source. The narrow frequency range of the phased locked loop (PLL) used in such applications limits the range of input frequencies of the signal to be measured. Also, to construct a PLL, the phase must be measured and the source frequency controlled. Also the PLL requires an optical pulsed laser source with tunable repetition frequency which is expensive.
In accordance with the preferred embodiment of the present invention, repetitive sampling of a data signal is performed. A clock reference is generated. The clock reference has a known period relationship with the data signal. The clock reference and the data signal are simultaneously sampled. The sampled information from the clock reference is used to determine in what phase of the clock reference sampled values of the data signal occur.